Learn about this topic in these articles:

classification and characteristics

The skulls of
iguanodonts accommodated still larger jaw muscles, but the cheek teeth were less regular and compacted than in the primitive ornithopods and consequently did not occlude as uniformly. Both the premaxillaries and the predentary were toothless but probably were sheathed in horny beaks.

type of ornithopod

...and longest-surviving families of dinosaurs. They ranged in size from 1.5 to 7 metres (5 to 23 feet) and show evidence of having been speedy runners. They may have given rise to the more advanced
iguanodontids and hadrosaurs, which spent most of their time on all fours. The
iguanodontids were medium-sized to large dinosaurs with the specialized grinding teeth typical of advanced ornithopods....